Why we can’t catch up with the mumps outbreak

New Zealand is in the grip of its worst mumps outbreak in decades. The outbreak started in Auckland early last year and was initially focused on five schools in the West of the city. Since then there have been over 1000 cases with some spreading to other centres around the country.

Mumps causes a fever, headache, muscle aches and most cases have swollen salivary glands, usually on both sides of the face. It is spread from people by saliva or mucous droplets spread when coughing. Occasionally it can cause serious complications like meningitis and hearing loss. Instances of swollen testes caused by mumps are now being reported in Auckland, a condition that can lead to infertility in men. Because it is caused by a virus, antibiotics are not effective treatment for mumps.

Auckland has been the location of recent mumps outbreaks. Photo by Mathew Waters on Unsplash.

An early effort to contain the outbreak in West Auckland offered vaccination to previously unvaccinated school students. This is called “catch up” vaccination, and it focuses on unvaccinated (or undervaccinated) individuals and assumes that previously vaccinated students will be immune. Despite this catch up campaign the outbreak spread beyond the schools, and public health officials started recommending catch up vaccination to young Aucklanders and others via mass and social media. The ministry also advises that people who received two doses of the Measles Mumps and Rubella (MMR) vaccine (as has been recommended since 1990) are “considered immune” and don’t need a further dose. But because vaccinated people can lose their protection from mumps over time, a significant minority of the people who have developed mumps in this outbreak are in fact vaccinated.

This isn’t a surprise. Small mumps outbreaks have been seen in vaccinated populations around the world. The vaccine is about 90% effective, and its protection reduces over time. That means even with high vaccination rates there will be some susceptible people and small outbreaks might occur from time to time. Normally, the vast majority of vaccinated people will never be exposed to mumps, and it won’t matter if mumps immunity has reduced, because they’re not exposed. Unfortunately people who are adolescents and young adults today had low uptake of MMR vaccination when they were children. So this combined with the expected reduction in immunity with age creates a perfect storm. Some estimate we have over 570,000 people susceptible to mumps virus and are at risk of an outbreak that takes years to burn out. While most people vaccinated in childhood will still be protected, many will learn they were in fact susceptible to mumps when they catch the disease.

This 1977 thin sectioned transmission electron micrograph (TEM) depicted the ultrastructural details of the mumps virions that had been grown in a Vero cell culture. Wikimedia Commons.

The risk of outbreaks of vaccine preventable diseases in young adults is well known to New Zealand’s health authorities. A 2014 outbreak of measles in Auckland identified young adults as a risk group due to low MMR vaccine coverage and a report in the New Zealand Medical Journal called for catch up campaigns. Catch up vaccination is funded by the government (you can go to your GP and be vaccinated for free even if you missed out as a kid) but the “campaign” so far has consisted of imploring unvaccinated young people to go to their GP, an approach that is bound to fail because it relies on young people remembering whether they were specifically vaccinated for mumps 20 years ago, and if not, getting themselves to a general practice. In December 2017, 11 months after the beginning of the outbreak, the Ministry decided it would initiate a school-based catch up campaign, but concrete plans for this massive undertaking are yet to be put in place. All of these interventions target vaccination at people who were never vaccinated or those whose vaccinations aren’t “up to date” and will miss people whose vaccine protection has waned. Failing to address this fact limits our ability to respond to control the outbreak meaning mumps will disrupt education, sports and community activities for young people for the next several years. Failing to communicate that previously vaccinated people are susceptible also risks undermining public confidence in vaccines. It is a particular problem when authorities are not straight with young adults who we’ll soon call upon to make the right decisions with respect to their own children’s vaccinations.

According to a December press release, the Ministry of Health is focused on keeping the outbreak contained to Auckland. In other words, Auckland now has a generalised outbreak that cannot be contained, and containment efforts need to shift to other regions. So understanding what factors led to failed containment in Auckland is a matter of some urgency for other centres. Are they specific to Auckland’s population or Auckland’s stretched public health work force? Or is the strategy of only offering catch up vaccination simply not up for the job of interrupting mumps transmission in Auckland or anywhere else in the country?

New Zealand needed to start a catch-up vaccination campaign for adolescents and young adults five years ago if we were serious about preventing this outbreak. But starting one during this historically large outbreak is too little, too late as catch up campaigns only protect some of the people at risk. Recent studies have shown that during an outbreak, young adults in high risk settings can benefit from a third dose of the MMR vaccine, even if they completed their two doses as a child. A study done at the University of Iowa, where students needed to show they had their two childhood doses of MMR to get admission, showed in the context of an outbreak students who received a third dose at university were 78% less likely to get mumps than those who only had the two childhood doses. As New Zealand students return to school and travel to tertiary institutions around the country the Ministry of Health needs to urgently fund and offer a third dose to these people aged 10 to 29 even if they have received two doses already.

Ayesha Verrall is an Infectious Diseases doctor and senior lecturer in Infectious Diseases at the University of Otago, Wellington. She tweets on infectious diseases @germ_theory.

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